Microsoft has released the production version of Service Pack 2 for Windows Vista and Windows Server 2008. Those of you who are lucky (cough) enough to be using Vista can download it now from the Microsoft Download Center. If you use Windows Update, either automatically or manually, the SP2 update should be offered within a few days (according to Microsoft), assuming your machine is configured adequately (see below). The MS Knowledge Base has an article describing alternate ways of getting the Service Pack.
To download the Service Pack itself (the same updater is used for Vista and Server 2008), use one of the following links:
There is one important prerequisite for installing SP2 on Vista, which is that Vista SP1 must already be installed. Apparently Microsoft packaged things this way so that the SP2 download would not be so big; as it is, the 32-bit version weighs in at about 348 MB. Windows Server 2008 was released after SP1 was available, and it already includes the contents of SP1. (If you need to get Vista SP1, there is a download link in the Knowledge Base article I mentioned earlier.)